Present turbojet engines are precision machines capable of efficiently delivering the large thrust required by today's aircraft. In order to achieve ever increasing thrust and performance demands, engine blades must rotate at very high speeds and in close proximity to stationary components. Rigidity in such components is necessary to avoid undue rubbing by the blades and the resulting engine performance deterioration. These stationary components are held in fixed relationship through the engine casing. In order to keep the engine casing rigid, bending moments therein must be minimized.
The integration of engines to aircraft requires some means of engine mounting. The conventional form of mounting is to support the engine casing at two positions, viz. forward and aft mounts. At least two engine configurations require more complex schemes. Exhaust nozzles which deflect the jet for the purpose of improved flight maneuverability or vertical/short takeoff and landing produce large transverse loads which result in large bending moments in the engine casing.
One solution is to move the aft engine mount nearer to the exhaust nozzle. When this is done, however, the relative distance between the forward and aft engine mounts increases. Under loading conditions, such as flight maneuvers or landings, this increased casing span between mounts will cause the engine casing to deflect. Although it may be possible to install a third engine mount, the result is to tie the engine to the supporting airframe structure so that the engine bends with the airframe. Alternatively, the engine casing could be reinforced to resist bending moments, but this results in an unacceptable weight increase.
A second engine configuration that presents engine mounting problems involves extended length exhaust nozzles. These nozzles attach to the rear of the engine casing and produce bending moments by their cantilever action. In addition to bending moments induced by such exhaust nozzles, undesirable clearance between the nozzle and airframe is necessary to allow for exhaust nozzle deflection.
One solution to the above-referenced problems has been to provide a third engine mount towards the aft end of the exhaust nozzle and attach the exhaust nozzle to the engine casing with a flexible joint. In effect, the engine casing and exhaust nozzle are separately mounted with bending moments eliminated by the flexible joint. A typical flexible joint employs a bellows for flexible motion between casing and nozzle in combination with either a single axis trunnion or gimbal type joint for providing support for shear loads and axial loads. These joints tend to be heavy and generally bulky.